Steel pickling process



2,762,728 Patented Sept. 11, 1956 United States Patent 'ice Wrought stainless steels are a well known class of materials including austenitic, martensitic and ferritic steels, all of which contain a substantial amount of chromium (at least about 11% and preferably 12-25%). Wrought 5 austenitic stainless steels are characterized by the presence of nickel or manganese and chromium therein in a total amount of at least 23% (and not more than about 50%), the Cr content being at least about 14-16% and the Ni and/or Mn content being at least about 7%. Pre- 10 ferred austenic steels for use in the invention are A1Sl type No. 301 (0.08-0.2% C; 16-18% Cr; 6-8% Ni; and max. 2% Mn) and chrome-manganese" steel (0.05-- 0.15% C; 16-18% Mn; and 14-16% Cr). The martensitic steels have 11-18% chromium and up to about This invention relates to the cleaning of metal surfaces, 15 nickel, an example being A151 yp 431 and more particularly, to the cleaning of wrought stain- 02% 15-17% and The less steel surfaces by the use of a specific pickling solution. felfitie Steels have atittut 11-27% Chromium, an example Particularly in the fabrication of stainless steel hub of a Particularly Preferred Steel use in the invention caps, trim rings and the like, where appearance is an being A131 YP 430 012% C and 14-43% essential feature, there is often encountered a certain 20 amount of difficulty in perfecting the cleaning of the stain- Most Preferably the Steel used is in Sheet femt less steel surfaces, for example, to remove oxides thereg and it is formed into Sheet-like articles Such as from. The pickling of certain ferrous metal materials is, huh p Whee1 covers, The Steel which has been of course, old; but the pickling procedures h etof d found to be most preferred for use in the instant invention in the art have left something to be desired in connection is the above identified chrome-manganese steel, al- With the aning f Wrough stainless steel, and particuthough the other steel above identified may also be treated larly, a specific type of wrought stainless steel known as With distinct advantage y the use of the instant Pickling chrome-manganese steel. The difficulty here appar- Solutionently resides in the peculiar nature and extent of oxide Although the Precise P y and chemical. Pheutuue' formation which takes place in connection with certain 11011 Which takes Place during the instant Pickling Operaof the metals other than iron which are present in these tiOII Cannot be fully understood. is believed that the stainless steels. In general, the stainless steels have very tremely active chemical combination of sulphuric acid high resistance to corrosion, but the oxides formed from and nitric acid results in effective removal of even the certain of the metals present in stainless steel are equally most chemically inert oxides, and this removal takes place resistant to chemical action in many cases and can be redown to the bare metal surface. As soon as the bare moved from the stainless steel surface only with difficulty. metal surface is reached, however, it is believed that the The instant invention is based upon the discovery of a nitric acid (coupled with the unique inertness of the stainparticular type of pickling solution for steels of this type less steel per se) dominates the chemical activity so as to which operates under specific conditions to obtain unrender the steel surface passive or inert toward corrosive usually superior results. This pickling solution not only action by the instant strong acid concentration. accomplishes pickling (viz: the removal of the unde-v The m untp sa i-Egg aeignwhieh g e d in the sirable metal oxides, carbonates etc.) but it also has the instant'aqueous pic mg solution may range from a miniunusual property of rendering the stainless steel passive, mum effective amount for cooperation with the nitric so that as soon as the undesirable oxide, etc., have been acid (namely, about 5%) to amaximum effective amount, removed from the surface of the metal further chemical which is about 0' l I iht of the aqueous action will not take place. solution. (As used herein fthe terms parts and per- It is, therefore, an important object of the instant incent mean percent by weight, unless otherwise desigvention to provide an improved stainless steel pickling nated.) the amount of sulphuric acid used is process. about 10-15%.

It is a further object of the instant invention to provide The amount of ni'tric acid which may be used in the an improved method of pickling wrought stainless steel, instant pic mg solu' ion ranges from about the minimum that comprises the step of immersing the steel in an aqueeffective amount for cooperation with the sulfuric acid ous pickling solution of 5-20 weight percent sulfuric acid (which is about 5%) to the maximum amount which may and 5-20 weight percent nitric acid at a temperature of be used (whic 18 ab Preferably, the nitric 120-180 F. acid is used in a concentration of about 8-l2%, and most Still another important object of the instant invention preferably about 10%. It has been found that it is preferable to use t e same or lightly mo is to provide an improved method for the icklin of re sul huric acid wrought staipless chrome-man anese austemti steel than the amount of nitric ct; an e preierred ratio tiahtaihihETlOS-G157o C, 16-18% Mn and 14-16% Cr, of sy p l h ur c acid tanifrigggid by weight is within the that comprises the step of immersing the steel in an aquerange of ab'out l fl th about Q22. The remainder of the ous pickling solution of 10- i weight percent sulfuric solution is, of course, water, a ough the invention does acid and lg yeight rcent n tric acid at a tern erature ,r -----4-- of 140-16015.

not preclude the use of various other pickling solution additives such as stabiliz s etc. The essence of the iner 0 ects, features and advantages of the invention i will become apparent to those skilled in the art from the stant invention, as ar as materials used are concerned, resides in the particular cooperation between the sulphuric following detailed disclosure of preferred embodiments thereof.

acid and nitric acid with respect to the pickling of certain The instant invention consists of an improved method stainless steels.

The concentrations of these two ingredients used in of pickling wrought stainless steel, that comprises the step of immersing the steel in an aqueous pickling soluthe solution are important and also the tem ature used for carrying out the pickling operation is important. It tion of 10-15 weight percent sulfuric acid and 8-12 weight percent nitric acid at a temperature of 140-160 F.

STEEL PICKLING PROCESS Edgar A. Hahn, Grosse Pointe Park, Mich., assignor to m}: Incorporated, Detroit, Mich., a corporation of e ware No Drawing. Application October 2, 1953, Serial No. 383,922

10 Claims. (Cl. 134-3) has been found that the temperature used should range from a minimum of about F. Mum of about 180 F. The temperature used, of course, contiomstantial extent the rate of the pickling action, but it is also found that particularly important to employ temperatures at least within the aforementioned operative range in order to accomplish effective pickling of the superior nature here obtained. a o t eferably, the pickling temperature is within the range of 140-160 F., whereat ideal pickling operation is obtained.

As hereinbefore mentioned, the chrome-manganese stainless steel is preferred for use in the instant invention. This is apparently the case because certain of the oxides of manganese, chromium or complex mixtures thereof, which are characteristic for this particular steel, have been found to cause a great deal of ditficulty in the pickling operations heretofore employed. For that matter, the pickling of all of the stainless steels has presented a number of problems; but is believed that the particular chemical combination of oxides or the like undesirable surface chemicals combined with the specific metal or metallurgical characteristics at the bare metal surface (including the carbon content and the metallurgical character of the carbon present) have made the pickling of chrome-manganese steel most difiicult.

The invention may be demonstrated using a piece of 20 gauge sheet material which has been formed into a wheel cover, this material being the wrought stainless "chrome-manganese austenitic steel hereinbefore described and having been subjected to one or more heat treating operations so as to obtain a slight but noticeable oxide formation or scale on the surface thereof. A solution of 12%% sulphuric acid-102a nitric acid in water, which is main awn-at 150 F. is employed as a pickling bath and the steel piece is immersed therein. Active removal of the oxides and other undesirable surface materials commences immediately and after a short period of time all reaction appears to cease. The piece is then removed and washed and examined; and it is noted that the surface impurities have been removed completely, but the metal surface is not corroded by the pickling bath. If dilute concentrations of the acids, such as 1% of each, are used little or no oxide removal takes place. If the nitric acid is used alone very little pickling action occurs; and if the sulphuric acid is used alone corrosion of the metal will be noticed in certain locations. As previously indicated, effective action may 'be obtained using the nitric acid and the sulphuric acid each within the rangeof about 5-20 weight per cent; but a much more effective corrosion action is obtained using about -15 weight per cent of sulphuric acid and about 8-12 weight per cent of nitric acid. Also, if the temperature is reduced below 140 F., to about 120 F., for example, the pickling action is still noticeable, but much more efiective pickling action is obtained using temperatures within the range of 140-160 -F. Increasing the temperatures to about 180 F. is a generally undesirable practice because of the tendency to release volatile matter from the pickling bath.

The pickling of other wrought stainless steel is carried out by the use of the same procedure, for example, using I comprises the'step of immersing the steel in a bath con- 4 sisting of an aqueous pickling solution of 5-20 weight per cent sulfuric acid and 5-20 weight per cent nitric acid at a temperature of -180 F.

2. A method of pickling wrought stainless steel, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 5-20 weight per cent sulfuric acid and 5-20 weight per cent nitric acid at a temperature of -160 F.

3. A method of pickling wrought stainless steel, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 5-20 weight per cent sulfuric acid and 8-12 weight per cent nitric acid atatemperature of 120-180 F.

4. A method of pickling wrought stainless steel, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 10-15 weight per cent sulfuric acid and 5-20 weight per cent nitric acid at a temperature of 120-180" F.

5. A method of pickling wrought stainless steel, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 10-15 weight per cent sulfuric acid and 8-12 weight per cent nitric acid ata-temperature of 140-160 F.

6. A method of pickling wrought stainless chrom manganese austenitic steel containing 0.05-0.15% C, 16-18% Mn and 14-16% Cr, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 10-15 weight per cent sulfuric acid and 8-12 weight per cent nitric acid at a temperature of 140-160 F.

7. A method of pickling wrought stainless "chromemanganese austenitic steel containing 0.05-0.15% C, 16-18% Mn and 14-16% Cr, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 10-15 weight per cent sulfuric acid and 5-20 weight per cent nitric acid at a temperature of 120-180 F.

8. A method of pickling wrought stainless chromemanganese 'austenitic steel containing ODS-0.15% C, 16-18% Mn and 14-16% Cr, that comprises the step of immersing the steel in a 'bath consisting of an aqueous pickling solution of 5-20 weight per cent sulfuric acid and 8-12 weight per cent nitric acid at a temperature of 120-180 F.

9. A method of pickling wrought stainless chromemanganese austenitic steel containing ODS-0.15% C, 16-18% Mn and 14-16% Cr, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 5-20 weight per cent sulfuric acid and 5-20 weight per cent nitric acid at a temperature of 140-160 F.

10. A method of pickling wrought stainless "chromemanganese austenitic steel containing 0.05-0.15% C, 16-18% Mn and 14-16% Cr, that comprises the step of immersing the steel in a bath consisting of an aqueous pickling solution of 10-15 weight per cent sulfuric acid and 10 weight per cent nitric acid at a temperature of 140-160 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,337,062 Page Dec. 21, 1943 2,347,742 Keene May 2, 1944 2,476,413 Heath July 19, 1949 2,569,158 Francis Sept. 25, 1951 2,662,814 Swihart Dec. 15, 1953 

1. A METHOD OF PICKLING WROUGHT STAINLESS STEEL, THAT COMPRISS THE STEP OF IMMERSING THE STEEL IN A BATH CONSISTING OF AN AQUEOUS PICKLING SOLUTION OF 5-20 WEIGHT PER CENT SULFURIC ACID AND 5-20 WEIGHT PER CENT NITRIC ACID AT A TEMPERATURE OF 120-180* F. 